# Re: eddy current with secondary coil

```Original poster: Bart Anderson <classi6-at-classictesla-dot-com>

Hi Antonio,

This reply might be a day late (didn't check emails at all yesterday).

Those particular numbers were quickly modeled, but I've made this
measurement before and decided to go ahead and do it again tonight (it only
took a couple minutes to set up the equipment).

I set up a counterpoise under the coil. I leave the coil completely set up
and short the gap. I use the planewave for the Fres measurement to the
scope. I drive the coil with the generator with the low-z amp (which has
just become my typical drive approach). The generator is adjusted for Fres.
C is measured between topload or top of coil and counterpoise. C will
measure 1/2 the capacitance. L is solved from Fres and C. There is some
loading due to the direct C measurement, but the results match the model
well and show the affect on L and C.

Since Javatc can model the change in L and C, I'll show measured and
modeled numbers.

The coil driven at Fres unloaded w/counterpoise in place:
Measured:                   Modeled:
418 kHz                     419.8 kHz
5.3 x 2 = 10.6 pF            9.87 pF

solving for L
L = 14.8 pF                 14.57 mH

The coil driven at Fres loaded w/counterpoise in place:
242 kHz                     239.12 kHz
11.57 x 2 = 23.14 pF         23.99 pF

solving for L
L = 18.7 mH                18.48 mH

I should note Ldc measured is 18.76 mH. Thus, at low frequency we shouldn't
see much of a change with this particular coil and topload. Also, my
primary is actually a turn out against the model, however, the position of
this outer turn
Now granted, there is meter loading which has an affect, but I think the
numbers show pretty well a change in inductance (and capacitance) at Fres.
The values are within the error of the model. I find it matches the models
and Paul's work very well. The test is easy enough to perform for many on
the list with similar equipment.

Take care,
Bart

Tesla list wrote:

>Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
>Tesla list wrote:
> >
> > Original poster: Bart Anderson <classi6-at-classictesla-dot-com>
>
> > On my small 19mH secondary,
> > the inductance at resonance drops to 14.3mH, however, when I add the
> > topload, the inductance at resonance increases up to 18.3mH.
>
>calculated from a model?
>The values seem reasonable, due to the effect of the nonuniform
>current along the coil due to its distributed capacitance. The
>effect is stronger without topload, as expected.
>
>Antonio Carlos M. de Queiroz
>
>
>
>

```